1. Field of the Invention
This invention relates generally to a pulsed fiber array laser and, more particularly, to a pulsed fiber array laser that compares the phase of optical leakage between beam pulses in each fiber to a reference beam so that the phase of each fiber beam pulse can be phase-locked to the reference beam to provide a coherent laser array output beam without having to measure the phase of the pulses themselves.
2. Discussion of the Related Art
Coherent fiber array laser systems are known in the art for generating a good beam-quality laser beam from an array of fiber lasers. In recent years, there has been a rapid development of array laser architecture for high power laser weapon systems to destroy or “kill” distant targets, such as ballistic missiles, cruise missiles, bombers or the like. Known coherent array laser systems developed thus far have sometimes been tailored for these applications, where the laser runs in a continuous-wave (CW) mode or quasi-CW mode. Short pulsed (<10 nsec) fiber array lasers have been less developed in the art. Applications for such a pulsed array laser system include target range finding, target speed detection, remote chemical sensing, and remote target illumination. In particular, the scaling of pulsed fiber sources to pulse energies in the tens of milli-Joule (mJ) levels is highly desirable, the more so if the high efficiency, waveform characteristics and beam quality of the fiber source can be maintained.
It appears that coherent beam combining is the most promising method for scaling pulse energy beyond that achievable from a single fiber, which is currently limited to a few mJ. Depending on the intensity of the beam desired for a particular application, the fiber array may include 10-100 fibers. Perceived advantages of a pulsed coherent fiber array laser system include a factor of two to three in the improvement of overall system efficiency, significant packaging benefits and waveform flexibility.
Known fiber array laser systems employ some type of phase control that aligns the phase of each of the beams in the individual fibers to provide the coherent beam. Typically, the phase of each beam in each fiber is adjusted in order to phase-lock each beam to a common reference beam. Known coherent fiber array lasers are generally CW lasers where each of the individual fiber beams is on for a period of time that is long enough to measure the phase of the fiber beams, and to adjust the phase of each beam to phase-lock to the reference beam.
There is an interest in the art to develop a pulsed coherent fiber array laser system for various applications, where each pulse in the beam may be only on for less than 10 nano-second, less than the required time needed for the known coherent beam combining phase-locking techniques. A pulsed coherent fiber array laser needs to control the phase of the beam pulses in each fiber in a very short period of time in order to provide the coherent beam. Currently, no pulsed coherent fiber array laser system is able to provide phase control of the individual beams fast enough.